Classics: The Living Dead

30082008

‘Classics’ is a category of posts highlighting research that has made a real difference to biodiversity conservation. All posts in this category will be permanently displayed on the Classics page of ConservationBytes.com

In my opinion, this is truly a conservation classic because it shatters optimistic notions that extinction is something only rarely the consequence of human activities (see relevant post here). The concept of ‘extinction debt‘ is pretty simple – as habitats become increasingly fragmented, long-lived species that are reproductively isolated from conspecifics may take generations to die off (e.g., large trees in forest fragments). This gives rise to a higher number of species than would be otherwise expected for the size of the fragment, and the false impression that many species can persist in habitat patches that are too small to sustain minimum viable populations.

These ‘living dead‘ or ‘zombie‘ species are therefore committed to extinction regardless of whether habitat loss is arrested or reversed. Only by assisted dispersal and/or reproduction can such species survive (an extremely rare event).

Why has this been important? Well, neglecting the extinction debt is one reason why some people have over-estimated the value of fragmented and secondary forests in guarding species against extinction (see relevant example here for the tropics and Brook et al. 2006). It basically means that biological communities are much less resilient to fragmentation than would otherwise be expected given data on species presence collected shortly after the main habitat degradation or destruction event. To appreciate fully the extent of expected extinctions may take generations (e.g., hundreds of years) to come to light, giving us yet another tool in the quest to minimise habitat loss and fragmentation.

[…] destruction means our species (or at the least our civilisation) may well already be in a state of extinction debt? I’ve read a few things recently on the environmental catastrophe about to catch up with us, […]

[…] in the State continues to decline. Species extinctions have occurred in the past and a further “extinction debt” still exists. There is no reason to believe that this trend will improve without a change to […]

[…] Thanks to poaching, Northern White Rhinoceros were declared extinct from the Wild about 20 years ago. They were even killed because of some stupid belief in Asia that white rhinoceros horns can cure various illness. Sudan was captured and removed from the wild in 1975. He was residing at the Ol Pejeta Conservancy in Kenya for nearly a decade and protected by armed guards every second. Sudan has left two females as his legacy, his daughter Najin and granddaughter Fatu, but both of them are incapable of reproduction even if they have a mate. It’s sad that northern white rhinoceros are extinct, they just don’t know it yet. It is like they have become The Living Dead. […]

[…] degradation of the past has not yet wreaked the havoc on species due to a phenomenon known as ‘extinction debt’6. This well-demonstrated component of extinction means that extinctions continue years, decades […]